why do some colonies appear with blank center? and can two colonies be fused?

Answer 1

Two colonies can fuse if they are of compatible type.

Answer 2

One possibility is that the bacteria in the center have already died OR migrated.  They use up resources in the middle / leave waste products, and the motile ones then swim away.

Yes, two colonies can fuse.  You're doing your serial dilutions to reduce that happening.