Any extra units should go with the Western force. Mount the bridging engineer and send him north with the cruisers, and mount the paratroopers and send them as scouts. Move out horse cavalry first to get a feel for the area, then send up conscripts, other infantry, armor, and artillery. There will be a major bottleneck if you don't juggle your units just right to keep the roads clear. This can paralyze you long enough to cost you the battle.
Speed is crucial.
Once you've pushed north a bit on both flanks, both battle groups will instantly run smack into the solid wall of the Mannerheim line. You're only going to pierce this at the two cities, avoiding the center. Use conscripts to draw fire from the pill boxes, then pour artillery on them and start to break down the cities' defenses with stronger infantry and armor. Each city has a 37mm anti-tank piece, which you'll need to clear with artillery and infantry, so your tanks can maneuver in peace. Take out enemy artillery as quickly as possible, and keep your own artillery fire up.