you can do it it's just not advised. 1. you would have 2 pumps pushing water into the same area and would be creating head pressure against each other and could basically lower the flow of the pump. 2 if one pump was to fail the other pump would simple push water back out into the sump and not into the tank. 3. the tank would drain back drain into the sump which would would happen anyway if both pumps stopped, but this would do if one pump stopped also. Thing to remember here is water will flow the path of least resistance. So by looking at the pinched output nozzles you created by heating the PVC and squeezing it and regular 3/4" pipe. Where do you think the least path of resistance is?..... Of course it's the wide open 3/4" PVC. So now imagine you go away for a weekend, week, etc. and one pump fails. You'd have no water circulation at the tank until either the failed side was closed off with a valve or the pump was fixed. That could be up to a week if your on vacation. Who ever is feeding the tank while you are gone might not even know there was a problem.
Like it was said earlier, you can do it, but it's just not advised. You can have 2 return pumps but the best and safest thing to do is have them not connected on the return side.
Does that make any more sense?