obviously you do it in the sequencer in a few button presses, that's what the editing suite is there for.

However if you want to know if there is a third solution using just the composer, the answer is yes.

Copy fill 1 to variation 1, increase to 2 measures and erase the second measure. Copy fill 2 to variation 2, increase to 2 measures, and erase the first measure. Copy the bass from variation 1 to accomp 5 of variation 2. Then copy measure 1 of accomp 5 in variation 2 to measure 1 of bass in variation 2. You now have in the bass of variation 2 the combined basses of fill 1 and fill 2 as you wished. Repeat for accomp 1, 2, 3 and 4. Drum 2 is usually empty so copy drum 1 of variation 1 to drum 2 of variation 2, then copy the first measure of drum 2 of variation 2 into the first measure of drum 1 of variation 2. Clear accomp 5 and drum 2, the combined fills will now be substantially complete and quite useable since drum 2 and accomp 5 are the rarely used tracks, particularly so in fills. Now copy variation 2 into the fill position of your choice.

If the drum 2 and accomp 5 tracks are used and you wish completeness you can copy an original to them but will have to finish the missing measure yourself with a quick realtime play (or step if you prefer) which will be easiest in these tracks anyway generally and you can choose the harder measure to copy and therefore the easier measure to complete.

That's why you use the sequencer!