Each unit may be paired with only one other unit in the same turn phase, but it may be paired with a different unit in another turn phase. In Miker49’s example, the tank may be paired with the mechanized infantry in the Combat Move phase, then be paired with the tactical bomber in the Conduct Combat phase. In Jeffdestroyer’s example, the tank may be paired with the mechanized infantry in the Combat Move phase, then be paired with either the mechanized infantry or tactical bomber in the Conduct Combat phase, but not both. See the Combined Arms sidebar on page 16 of the Rulebook.