Question

What would it mean if there was growth in the two -plasmid plates but not in the two +plasmid plates

Answer 1

Although growth of two plasmids in the plate is also not a normal condition or usual thing and you may see growth in plates if these two plasmid don't belong to same replication site or origin in their sequence and technically both are different to each other. Moreover, you may be observed different sort of colonies on the plate means a mixture of colonies and two plasmids can exist if they express significantly high degree of compatibility which is almost impossible in the case of a bacterial cell with more than 2 plasmids, as a result you will not evident any growth in the plates, or not colonies due to non survival of bacterial cells. Mainly one type of plasmid can expect in bacterial cells.

Note: Please note that here we have discussed different plasmid, not identical one as single bacterial have several identical plasmids.