Thanks Scott, I appreciate the advice. I got a new bulb for the 25 as the previous was burnt out so no problems there. At present I don't have the parts I need to put them in serial; I've gone for a half way solution using an existing powerhead and returning to the sump. Project over the next week as I get the parts I think - I'll have to redo all the plumbing.The common opinion is to size the unit at 1 watt per three gallons so 40 watts or so for killing dinos. 15+25=40 which is helpful. I'd run both.
It is further suggested that more contact time is helpful; closer to 300gph. Lastly, you are strongly encouraged to run to and from the DT.
So given all that here is MY opinion. While aesthetically this is not a crowd pleaser, if possible you should run them serial to/from the DT, powered by a smaller (like MJ1200 lph) pump unless the manufacturer lists a minimum flow rate which is higher.
Lastly, these bulbs definitely age poorly. And if they've ever been powered on without flow they overheat in minutes. Don't ask how I know this . You can give them a try as is, but I would order replacements anyway, unless they look like unused units.