Hi Russell,
The planer is used to create a reference, or face side to a piece of timber by using in and out feed tables. The section of wood, just planed, rides onto the outfeed table and it is this which creates your reference face. A thicknesser won't do that, as the timber is just sandwiched between the table and the cutters, so if the timber is bowed you will still have it bowed, albeit nicely planed and even thickness.
One way round this is to use a sled, a trued up piece of timber, dead straight. Put your workpiece on top of this and feed the pair through the thicknesser. You may need to create an end stop to prevent the two sliding over one another, but light cuts usually do the trick. Once you've got one face 'true', you can dispense with the sled and use that face as your reference on the table. If you want the sides truing up, repeat the same process for them.
If you are using relatively thin pieces of timber, you may need to shim the work to the sled, otherwise it will flex and defeat the object of the sled.
I hope that all makes sense. It's much easier to show it being done than describing it!!