If you have two 150 RMS watt subs on the same ch of an amp What is the RMS watts needed from the amp?

Depends on the impedance of each speaker. If they are car subs and not converted home speakers, they are probably 4 Ohms each. Depending on how you wire them you get either 8 Ohms or 2 Ohms. 8 Ohms is not a problem load-wise for the amp, but if it is rated at 100 watts into 4 Ohms, then you will only get 50 watts into 8. Many car subs nowadays will handle a 2-Ohm load, but some will not. Your two speakers will handle 2x150 watts or 300 watts. An amp that is rated to drive 300 watts into 4 ohms will either deliver more like 500 watts into 2 Ohms, or it will melt through the floor of your car. So you need to look up the specs for your amp in the instruction manual. If it will only run into a 4-Ohm load, you can only use one 4-Ohm sub per channel. If it will handle a 2-Ohm load, you will have to turn it up slowly or you will blow your speakers out. Mr Ohm and his law will not be denied.