Would merging two coins' network into one to prevent 51% attacks mean the difficulty for mining any of the coins would also be merged (hence increased, as it's all added together)? If two networks merge, will there even be the possibility of mining only one of the two (or more) coins or would it mean you can only mine all of them at once?
It's a very interesting concept indeed. As the posts above said, most small hashrate coins are successfully attacked because the team behind them don't give a damn about protecting the coins. I've had absolutely no respect for XVG especially after their stupid and cringy PornHub & McAfee move.
It's best to avoid cryptocurrencies with negligent developers behind it, since you'd risk getting your funds manipulated easily across the Blockchain. Verge is a good example of a "useless" coin with no future, because of its negligent developers. They could've easily avoided many 51% attacks on the Blockchain, yet it's the date where it lacks a real solution to the problem. You get nothing by having a variety of merchants accepting the coin, if the same lacks security above all else. Remember, that it's all about censorship-resistance/decentralization than convenience.
Merging two or more coins will definitely increase the mining difficulty of underlying blockchain networks. It's beneficial for all sides as blockchain networks combine into a "single chain". When we combine small blockchain networks with medium-to-large ones, we get unparalleled security against 51% attacks. Luckily, we have many solutions on the Blockchain industry for cryptocurrencies with a small network hashrate. Solutions like VeriBlock, dPoW, Merged Mining, and even a multi-PoW consensus algorithm (like it's the case with DigiByte and Myriadcoin) are able to "tackle" the existent problems of a 51% attack. The problem lies on the developers themselves than anything else. If they don't care about security/decentralization, the coin can become an easy target for hackers. It's best to avoid any cryptocurrency which doesn't focus on security to prevent undesired losses.
Speaking from a trading perspective, at one point of time in 2017 Monero was good to be considered as an alternative to Bitcoin in terms of anonymity and using the CryptoNote algorithm but practically speaking almost all the altcoins are losing its potential to withstand the market. There has been various de-listings of XMR from exchanges and there aren't hardly any news to support the stability for a longer run.
Though Bitcoin is harder to mine with the current difficulty they re-adjust based on the price actions. When a miner feels like he is no longer able to run into profits from his mining activity, they would be leaving the network for other miners and subsequently we would be seeing a drop in difficulty.
But ASIC resistant on the other hand could be a positive move from the XMR dev team to make the coin more decentralized. ASIC mining would make the coin community rely on a particular company to mine their coins thereby encouraging centralization. We should be very well aware of CZ trying to manipulate the whole network by hiring miners to reverse his binance loss from the hack. If a rich person tend to change the working of a currency to the way he likes, isn't considered to be unhealthy for the whole market and the coin?
Monero has experienced various de-listings of exchanges because it's a privacy coin itself. Considering that governments are requiring ID verification within centralized exchanges, privacy coins like Monero and Zcash are much harder to track and enforce KYC/AML regulations. After all, transactions are obfuscated from the general public (especially Monero). The excuse of governments is that privacy coins are used for malicious purposes like money laundering and terrorist financing. But the truth is that most criminals use Fiat instead because it's stable and accepted by anyone in the world (not to mention that it's also anonymous in its physical form). As long as governments are against privacy coins, we'll continue to see exchanges de-listing them as time goes by.
Speaking of ASIC-resistance, Monero seems to be doing a pretty good job. This is great for decentralization, but constantly changing the algorithm every 6 months will make it harder for miners to keep track of the latest changes on the Blockchain. They'd need to change their devices (depending on the algorithm used) every 6 months which is not very convenient, in my own opinion. AFAIK, Monero hasn't experienced a 51% attack yet so I think it'll do fine for many years to come. The real issue will be unpopular coins on the market with a small dev team and community. If they'd want to survive for a long time, they'd need to adopt one of the aforementioned solutions to protect themselves against 51% attacks. Otherwise, people will lose trust in small coins day-by-day as they're constantly targeted by hackers in cyberspace.