For DBA, the key thing is that every unit (infantry and cavalry) has the same frontage. For 28mm, it's usually 60mm. Depth is less important; there's 20mm (solid blades and spears, etc.), 30mm (warband and fast blades) and 40mm (hordes and cavalry), but as long as the depth fits the figures well, it doesn't really matter.
Impetus uses 120mm and also has every unit with the same frontage. So two DBA elements gives you the correct frontage for Impetus.
I'd recommend using the DBA element sizes for three reasons.
First, if you want to play DBA, distances and table sizes are connected to frontage sizes. To play DBA with 120mm frontages, you'd need a much bigger table than the standard 3' x 3' (and even a 6' x 4' table would be too shallow; you'd need a 6' x 6' table to scale the game up).
Second, it's easy and sometimes helpful to combine several smaller bases in one unit. You can always use a strip of masking tape to hold bases together temporarily when playing Impetus. Also, To the Strongest (a great game!) works really well when multiple DBA bases are used for a single unit; you disarrange the elements to show that the unit is disorganised.
Third, painting up a DBA element or two is a nice, manageable task. It doesn't take long to paint up an entire 12-element DBA army, and then you can expand that into an Impetus army by adding more elements. Painting big bases is a bit more daunting (although they do look terrific).