2x15w = 30w/40gal = .75 wpg
Wpg isn't used very often but even so you'd have pretty low light. Might not be able to grow much other than the very low maintenance plants.
Lots of different options to go with nowadays with T5s and LEDs becoming more of the norm with planted tanks. Hence why Wpg is becoming outdated because it doesn't work well for more efficient lighting. I wish I could give you a more specific suggestion, but I haven't chosen lighting for a planted tank that size.
Have you read Hoppy's thread about PAR? It's a lot of text, but a good source of info.