Multiple items on SDA/SCL pins? Mega 2560

[magentaline]

@anon35827816

This is the only instance in which you cannot change the pins (easily). In all other cases - it can be done.
...why not tell a bit more about this mystery application that blocks pins 20 & 21?

Perhaps the OP's arrangement includes a shield, with some stuff hard-wired into those pins. (shields are a pita...)

Tell all, OP....