Yes, given how rare it will be, I'd say better to rely on common-sense or an umpire to work out the details.
I was going to use this for my Vikings. But I'll still buy at least 4 longships to protect him from any enemy naval.
If I was umpiring I'd say that the smaller (friendly) command comes on shore first with impressed shipping and dices as normal. Unless the enemy has another command with naval, then the score would be +2 (assuming no fog/winds). The General and his fleet arrives as well. PIPs will be scarce, but hope for high PIPS as you will need them to turn the naval around. Next bound the larger (enemy) force arrives. If you are smart, you'd take at least 4 naval because then the enemy will need at least 8 to avoid a score of +4. This may end up as a disaster for the enemy as he will have a lot of spent troops, plus YOUR naval should immediately attack HIS.
Why is your score +2 and his +4? Because the rules say "by 2 if enemy naval present exceed half". Present. At the time of the friendly arrival, the enemy have yet to arrive and are not present. So unless the enemy have yet MORE naval on the table, you only need to score +2 and if you have bought enough naval, he will need +4.
So this means, if you are going to do this, you really need to buy as much naval as you can afford. Plus if you have a Sea or Waterway, you may need to buy naval to protect you against a flank-march now we have a mechanism that reduces the risk to the General. I've never used because I thought I'd have to dice for the General!
Cheers,
JohnG