I am running butane and have a big ol’ supply tank to keep cold. The chiller on it is woefully inadequate but we have a couple other chillers laying around the lab unused. I was thinking of running them all together and seeing if that won’t improve the system’s ability to remove heat.
Any one ever done this? Did you put them in series or in parallel? Did it even work?
What chillers do you have? In theory it can work but depending on the chillers you have and the temp you are trying to stay at. It may or may not be worth it. How big is “big ol’” 
I’ve done it, not on a butane system but on a falling film with a ten ton and a 20 ton glycol chiller for the condensing side.
Series vs parallel is the wrong way to look at it, what you want to do is have one feeding your process and on the return put a heat exchanger or multiple heat exchangers that you can tie your second or third chiller into. If you try and do series or parallel you are practically guaranteed to overflow one of the coolant reservoirs, even with identical chillers small changes in hose lengths or manufacturing tolerances will make for unbalanced flows
You can gravity feed from an added “overflow” into the bath if the second chiller to avoid overrunning (and only need one pump). Pro(hack)tip
Closed reservoir chillers are easier