Ian...I have no experience with two twin cylinder steam engines with a single boiler, lubricator & steam regulator....but thinking.......
1. We are assuming that the pair of engines were manufactured from the same production run or purposely machined/toleranced from kits at the same time
2. From this we also conclude each engine would consume near identical volumes of steam & steam oil for any given load
3. I know that ANTON will build mirror reversed versions of his engines....this way for twin engine builds the all important identical length, shape & form of steam inlet & discharge tubing can be maintained between each engine
4. As an insurance.......your member build could include a needle valve identically located in the steam inlet line just prior to each engine for discrete throttling if needed
5. We must also remember that each engine will be rotating in opposite directions........I do seem to remember a thread here on Mayhem detailing or asking why the members TVR1A was running at different speeds between ahead & astern..........Pettyofficernick ...may be able to offer comment here..............Derek