What's "a lot more" in percentage terms?
Are both engine
hours going up by the same amount between fill ups?
Are you running them one at a time or both together?
If both together, are you sure they are both running at the same RPM
? Are you judging by a rev counter or by tuning to "beat frequency".
Rev counters can easily be 1-200 rpm out.
Do you have an extra drain on one engine
such as a large alternator
, eutectic freezer compressor