What is the interaction between Null Profusion and Reliquary Tower if I control both at the same time?
It all comes down to timestamps.
When multiple effects are trying to set or modify your maximum hand size, then the most recent effect of that type wins.
So, if Null Profusion enters the battlefield after Reliquary Tower, your maximum hand size will be 2. If Reliquary Tower enters the battlefield after Null Profusion, you won’t have a maximum hand size.
Effects that increase or decrease your maximum hand size by a specific number modify the most recent effect to set your maximum hand size as long as they’re newer than that effect. (They’ll have no effect if you don’t have a maximum hand size, though.) A more recent effect that sets or removes your maximum hand size will override these increase or decreasing effects.
For example, if you control a Minamo Scrollkeeper and nothing else is setting your hand size, then your maximum hand size will be 8 in a normal game. If you then play a Null Profusion, your maximum hand size will now be 2 since Null Profusion’s effect overrides Minamo Scrollkeeper’s effect because it has the newer timestamp. If you then blink or bounce and replay Minamo Scrollkeeper, then your maximum hand size will be 3 as Minamo Scrollkeeper’s timestamp is now more recent that Null Profusion’s and its effect can modify the effect from Null Profusion.