Different services use different seeds a lot of the time, but if you have two services that are compatible with the same kind of seed, it's perfectly fine.
That said, you have to realise that by using your seed in multiple wallets, your coins are as strong as their weakest link. For example, don't use a hardware wallet seed in an ordinary software wallet on your PC, because it would defeat the point of using a hardware wallet.
This makes it pretty much pointless to try having a seed in two places, because the main purpose of doing it would be convenience (for example, you would have one wallet stored very safely and one that is convenient for everyday access). You might as well just use different wallets.
Is it possible, say, have the same seed for different Electrum wallets? For example, I have BTC, BCH, VTC, etc Electrum wallets. Can I use the same seed for all of them?
BTC and BCH use the same address format, so I think the same rules apply as long as you're using a legacy wallet. I'm not familiar with VTC so I can't tell you, but I doubt it.