a gas engine doesn't pressurize the tank, it draws fuel from the tank using a small pump in the carb,you need to make sure the tank has a vent to allow the engine to draw the fuel.

a typical vent line wraps around the tank a couple of times then exits the bottom of the fuselage, you give it a couple of loops so fuel doesn't leak out of the line, as long as the tank can draw air to replace the fuel being used,unless there is a inordanate ammount of bubbling of fuel or a air leak in the fuel line than its not going to cause engine stoppage.

most of the time its overheating that causes a gas engine to quit,has it been broken in or is it well used,if the engine is new it needs to be properly broken in just like any engine to seat the moving parts,if its used then there might be a clog in one of the jets or passages in the carb causing the stoppage.

piggy backing tanks is fine just hook up the slave tank supply(the line with the clunk) to a strait pipe entering the primary(the one hooked to the engine, with no bend entering the primary tank,you only need to vent the slave tank as it runs out of fuel its vent allows the fuel to drain from the primary.