You can but I doubt you would see a significant improvement unless your system is bogged down with multiple blocks and rads. If you run them in series, I would make the L20 the first in the series but might not make a difference either way.
seems to me that running them in series, one right after the other could actually hurt water flow. i mean the l20 doesnt pump as much as the l30... so either the l30 is gonna be shorted on waterflow, or the l20 is going to restrict the flow from the l30. now if you put them on opposite sides of your flow loop (so to speak) with a bunch of blocks and multiple radiators, then you might see some improvement. i dunno.
I'd use both... The L30 isn't going to be able to keep up with all that restriction assuming you're using a WW/RBX/TDX/6002/6000 WB... Even 3 low restriction blocks would be pushing it IMO.
BTW, my MCP600 may push half the water L30 pushed at zero restriction, but in a single block (WW) loop, it sure pushes a heck of a lot more water than my l30 ever did... The L20's flowrate compares to the MCP600, and since the MCP600 works well in a restrictive setup, I feel the L20 will only add to performance levels, NOT subtract.