That would be a solution, but in the army list book, Republican Romans are small units and they can double up against standard sized units, getting 10 dice versus 7, not counting supports. A solution is to make the small units wider so that 1 1/2 times their width is greater than the width of a standard unit. Per the chart on page 16, small units would have to be 120mm and standard units no wider than 170mm. We need to know if the designers intent was to have smaller units doubling up on a standard unit, the trade-off being more fragility due to lower stamina.
Actually, the rules only give those numbers as recommendations. You don't have to quite follow them. I mean, my Roman cohorts are all 24 figures strong all based on 40x40mm bases, so 4 figures to a base and 6 bases to each cohort. However, I count my first cohort as a large unit, because it was much bigger than the others and I have the figures mounted on 60x40mm bases with 6 figures to a base but keeping to 6 bases for the cohort.
Now, I don't know how many figures most people will be using and I agree that if everyone uses the exact multiples suggested, then two on one situations will be the norm. But from what I can gather, most seem to be using a similar basing system for their forces, which means that it would be difficult to get 2 standard units against the large unit I have mentioned above. The photos I have seen posted up by most members on this forum, or in blogs, appear to be basing their units in a similar fashion to the one I have mentioned above.
In our small group we have had to tackle this problem as well when we fought the Roman invasion of Briain. Initially we felt that the ancient Britons appeared too organised in standard sized units, especially as the players tended to deploy them as though they were legionary units, and so fought one on one. So we introduced a house rule to require that if a British division included more than two standard sized units, then two of them should merge into a large unit and this introduced the diversity we felt was historical. We did not insist on 400mm but used 320mm, which is the allowable minimum (page 16 of the ruleset). Oddly the larger units were not as vulnerable as we feared as they had a higher shaken value and often saw off the Romans. However if a lone large warband encountered two legionary cohorts with support, then it died. Probably historical though.
For republican Rome, we are introducing house rules to reflect rules in WAB Hannibal army lists for "manipular formation swap". This allows the smaller units in combat to be replaced by supporting units (whilst in combat). If this makes them too powerful against Carthaginian forces in Hail Casear, however, then we will need to revisit. However, we have a few more unit bases to construct and some painting to do before we test that one.
In all the above, note that it is the unit frontages which matters not the number of figures. For figures, we have retained the WAB base sizes except that we base skirmishers on 25mm circular bases and open order light infantry on 25x30mm to show some difference between close order light infantry on 20x30mm. Otherwise we forget the difference during the battles.
In our group we use 200mm for standard units & 100mm for small. So the Republcan Romans can get 2 units v. 1 enemy. If they do so, they have an advantage. If they have to fight one-on-one they are at a disadvantage. Points wise, 2 small legionaries cost 2x23=46 pts, 1 standard legionary costs 32 pts. All those things considered, both seem fair value, you just have to use different tactics reflecting the difference between the manipular & the cohort organisations. So what's the problem ?