I would not think so.

Unless your attacking army was a full stack army I would have merged both armies and then just leave the "garrison army" behind after the battle.

Sure the second army may take some casualties but they are just for garrison anyway. Plus the extra number could come in handy if the battle goes a little "pear-shaped" as they sometimes do.

Even if the first army is a full one if you place the second one in the "redzone" around the ciry and you can use them as reinforcements if plans do not unfold as desired. (stray onager shot killing your general etc.. )

The only time I would not use the garrison army with the attacking army is if it was a move behind the army. Thus you take over the town, on the next turn the garrison army makes it to the city and the attacking army heads off for plan "B" of the conquest/blitz.