Its best to plumb the two drains into the sump seperately. This way if something clogs one it wont have a chance to clog the other. The tank should come with bulkhead fittings for the tank overflows. From there I would use two true unions. This way you can disconnect the sump plumbing from the tank if/when you need to move the tank and you wont have to cut your plumbing up to get the tank off the stand.
From the true unions I would run them into either the same side of the sump or to opposite sides. If you go with a comercial sump/wetdry then you will have to plumb them to one side of the sump where as if you built your own sump you could design it to accept water on either side. Try to design your plumbing to allow for a decent down angle on the side that has to traval the furthest. To much of a horizonal run will end up impeeding the performance of the overflow.
From there they would dump into a drip plate for a wet/dry or would dump directly into the sump.
There is no real rule of thumb on sump size expecially when talking about power outages but there is some level of logic to picking the size. For example a 20 gal
sump would be to small for a 150 gal
of course). This is because you cant run the sump at full capcity when the pump is running. You need to account for power outage and account for back syphon when the pumps off. So a 20 gal
sump might run with only 5 gal
of water in it so it could accept upward to 15 gal
of backsyphon and not overflow. Using a 55 gal
tank like I iniallty suggested would let you have say 20 gal
of water in the sump system and still have 30 to 35 gal
of reserve capacity to accept backflow. You can reduce the amount of backflow by adding a one way 'flapper' valve to your return plumbing. This valve prevents water from flowing back down the plumbing so when the power fails water drains into the sump until the water level is even with the bottom of the overflow inlet slits.
I would affix a true union directly after and before your pump aswell. This way you can remove the pump with out having to cut it out of the system and then have to do some kind of repair job on your plumbing. A ball valve is good to put directly before the true union going into the pump and directly after the true union going out of the pump. This way you can cut off the water before removing the pump. If you dont do this you will have to drain the entire sump before servicing the pump.
To plumb the return lines just pick how you want the returns to work. Do you want one return in the tank or do you want multiple returns. If you want multiple returns I would suggest you look at the SQWD product as this is an alternating current device to facilitate two returns to the tank.
For more info on sumps in general check our articles area. In addition you might be interested in a Sump/Refug combo.