Yes and no.
No, you cannot directly connect two pumps to the 3155 board. The board can only supply 1.2amps and each pump takes almost a full amp to run. At least that's what I'm reading from something
@rvitko wrote when I was looking for specs. What I don't know, however is if slowing down the pump would result in less amp draw. I also don't know if drawing too much from the board results in a damaged board or blowing a user replaceable fuse.
Having said that, what you CAN do, is pick up a switched outlet.
This one can provide nearly 8 amps (but pay attention if buying it, it looks like they either have multiple styles for different countries or require adapters). If you're not familiar with this, you plug the wire that goes to the pump into the outlet. The controller simply turns the outlet on and off. The idea being that you can than use any pump you want which is useful if you need to push the water up higher than the included pump is capable of. However, you could also use it (with an outlet splitter) to attach multiple pumps.
Just keep in mind, that each pump will need it's own line running up to the tank or they'll need check valves between the pump and where you connect the two lines. Otherwise if one pump dies, the good pump will push water up to the splitter and it'll just fall back down though the non-working side.
Having said that, this is a hundred dollar project between the pump and the outlet. Before I pulled the trigger on that, I'd look into finding or making my own switched outlet with a 12v relay (or whatever the 3155 puts out). Either that, or at least consider just using a second Tunze 3155. It would only be an extra hundred bucks or so (above and beyond a switched outlet and pump) and that extra hundred bucks gets you an additional controller. So not only are you protected against the pump going dying, you're also protected against problems with the controller. Furthermore, if you set the second sensor an inch or two below the first one, it will only run if the first one is having problems. Meaning that one the first one dies from general wear and tear, the second one won't be right behind it. It should still have years and years of life left.
That'll also give you redundancy of the optical sensor which, quite commonly, gets dirty and fails to notice the water level falling. Since the second sensor is always submerged it shouldn't get full of as much muck and salt creep.