Buying same stock at same price and time from 2 different brokerage firms

There is no legal limitation to trade in as many accounts as you want - but there is nothing to gain for you by splitting it in any way between accounts.
For taxes, all accounts are considered one - so wash sale rules apply across accounts.

The only difference is that you have to manage it all yourself, and that can quickly get extremely complicated. If you hope that it gets so complicated that the IRS doesn’t catch you - it would still be illegal to hide it from them.

A wash sale violation occurs you acquire a “substantially identical” security or option within a 60 day window around the date that you realize a loss. That means within 30 days before and 30 days after the realizing that loss.

It makes no difference whether you do this in one account or the various transactions occur in different accounts. In the eyes of the IRS, trades in multiple accounts are considered as if they were done in a single account.