I agree with ccj. you should be able to start the 6 easily by pushing it. advantage of having another dozer is that you can get to lift the blade a bit on the 6, reduce the drag and get some traction on the 2. We started our d7 for years by towing it backwards with the blade dragging, have even done it with a 2 wd drive tractor. Use of the decompression lever is essential to get it spinning over.
Cat D7 4T; Cat 951 A
Fowler VF crawler
Aveling Barford DX8 roller
Ransomes TS 69 hexatrac plough (2)
Ransomes mole plough