Yeah at 1mph and a 1/16 jig you are only going down about 6-8' with 30' of line out. Using double 1/8 you should get down there, or one 1/8 and one 1/16.

Are you fishing in 15' of water or do you want to be down 15' over really deep water? Take your boat into 15' of water and toss out one rod and start dragging it around with it in your hand at your normal speeds. Use different weight combos to see what drags bottom and that will tell you what you need to fish 15'. It's really just trial and error to find what works best in your situation, depends on any current (wind current too!), how much line you let out, the dia of your line, the resistance of your jigs (big plastics vs hackle jigs etc), and angle of your rods even. You just need to keep going heavier until you are too heavy, then back down a little bit. Using two jigs will really help dial in the correct weight, but remember water resistance of the plastic/minnow does play a part... so two 1/8 will not go as deep as one 1/4 for example.

I know I fish a lot of 12' of water and where I fish a 1/8oz weight will let me just tick bottom at 0.5mph, at 0.8 it starts lifting and at 1mph its about 4' off.. so 8' down or so.