Recently, I had a transaction which is stuck until now because of low fee. What if I do another transaction with high fee, will it speed up both transactions? or just the one with the high fee?
Not exactly, having low or high fee of transactions does not affect the speed of the transaction at all. It is just a network disruption, now don't panic that you might end up losing it because you won't. Hopefully the lightning network will do its job in times like this, or more like will it really be implemented in all platforms.
If it will continue to be stocked, eventually it will probably just going to be returned in your wallet.
And doing another transaction overlapping your unconfirmed transaction is not recommended as it may result an even slower transaction time, if not then worse.