Doug,
The Bowser boiler would be way more than enough for a double K-27 Garratt. I'd remove a course from the boiler to shorten it up. Cut it at the smoke box and the next section back. Also, the 2-10-0's had shallow fireboxes, since they were located above the rear drivers. On a Garratt the firebox can go all the way down almost to track level. You might want to make up new firebox side sheets to reflect that.
I bought one of the Westside American Garratt's and destroyed its collector value by rebuilding it. The pivot points for the boiler bridge on the engine units are all wrong. I also had picked up a boiler from an old Balboa K-36 that someone had dropped and broken the mechanism. That gave the boiler some heft, although even it wouldn't be enough to power two K-27 mechanisms in reality.
Michael Allen