Can SQL Server use multiple indexes for the same query?
Yes, but not with the indexes you've chosen.

If I create these indexes:

CREATE INDEX ix_top ON #ab (col1a) INCLUDE (colb);

CREATE INDEX ix_bottom ON #ab (colb, col1a);


I get back this plan:



And that's without having to use multiple index hints.

With a slightly different index:

CREATE INDEX ix_mindyourbusinessypercube ON #ab (col1a, colb);


The plan changes to use just that one index, though performance ramifications are difficult to surmise given the limited test data. I leave that as an exercise to ypercube the reader.