What happens if I stake two same wallets (share the same wallet.dat) on two different machines at the same time?
I've done it in the past.
Not a good idea. Nothing bad happens but the balances get out of sync somehow after a while and you need to "fix wallet" to make them accurate again.
If only one of them stakes and the other is read only I think there is no problem. You don't stake faster anyway with 2 machines