Can I have the same wallet opened in multiple computers? I wanna put my wallet.dat to another node, and keep them online at the same time, Can I do like that?  and is there any problem?

BTW, how can I  do load balance with bitcoin node?  Thx
Provided that your wallet is an HD wallet (Bitcoin Core will show you a small HD  in the bottom toolbar if it is), then you can safely use it on both machines, as addresses are deterministically generated.

If your wallet is individual key based, then after you copy the wallet to the second machine, they will diverge and generate different keys for future addresses.