If a 5-year-old child has never received any doses of MMR or varicella vaccine and now the parents want him to catch up with the combination vaccine MMRV, what is the spacing requirement between the two doses?

Twelve weeks. The spacing between doses of a combination vaccine depends on the longest minimum interval of a component. The minimum interval between doses of MMR is 4 weeks; the minimum interval between doses of varicella vaccine is 12 weeks for a child this age. So, you should wait 12 weeks between the doses of MMRV for the two doses to be valid.