When a bridge or tunnel is signposted with maxheight on the lane + maxheight in the middle it is surely allowed/possible to drive your truck in the middle of both lanes, as otherwise it would not be signposted.
There is a short tunnel in my area with similar signposting and I used the value for the middle as maxheight, because this value represents the maximum possible vehicle height allowed to pass, and this is what is relevant for trucks, buses and the like.