What is the difference in 64bits TWRP and 32bits TWRP? I am new to android.
Can someone tell me what is the difference between 32bits TWRP and 64bits TWRP?
What would happen if I mixed up both TWRP?
I think you are probably using a Xiaomi or Moto phone hence the question. Well, most Android devices stick to one Architecture either 32 bit or 64 bit. But some OEMs push out 32 bit OS on 64 bit SoCs (like Moto and Xiaomi). The OS needs to be compiled separately for both these memory sizes. In Moto's case they stick to 32 bit throughout the device's shelf life, so you'll only find 64bit custom Roms, and they need 64 bit recoveries. For Xiaomi they did upgrade few 32 bit devices to 64 bit, so for flashing newer updates you'll first need an updated bootloader + stock recovery of 64 bit. 

TL;DR - If your custom ROM mentions "64 Bit" anywhere, use 64 bit recovery, else stick to 32 bit.