Not an expert, but I see no reason why this would not work. Your bandwidth would be limited by the frequency of i/o writes and number of i/o pins you use - in the gigabit/sec range, but still lower than something you could achieve within a single fpga fabric. Reliability and lack of shielding using extraneous wires might be an issue at high frequencies, but also might work fine. Buying a bigger FPGA, using external (non-fpga) ICs for simple tasks might give you more LUTs to play with. What type of project are you working on?
You can definitely do this but the biggest issue you can run into is timing. If the two FPGAs are running off different clocks, your signals will inevitably get out to sync. If you run them both off the same clock (it’s possible but kinda tricky to output the clock to another pin) you can still run into issues depending on how long the lines are.
If you just want to use something simple like UART or SPI to talk from one to the other that’s pretty easy.
If it’s possible, it is usually better to go with a bigger FPGA. Then the tools take care of all the internal timing for you.