Hi,
This is the first time we've made a cake like this. How would we go about stacking this? Does it need supports? What kind? Its a first cake that is for a company and needs to be perfect!
Any tips would help. Thanks so much!

Any cake with more than 1 tier needs to be supported with some type of support system or the upper tier could crush the lower tier. The type of system is up to you.

I've done this combo many times. I keep a template of the size of a trimmed 1/4 sheet board. After you ice your 1/2 sheet, putit in the frig for icing to get cold/hard. Take that template and lay it on top of the half. Make sure it is centered. Use a ruler to make sure. I then take a pin and trace it out. Then I add my dowel rods. 3 on the sides and one each end (total of eight). After you ice your 1/4 sheet and have it trimmed put it in the frig. Stacking cold cakes is much easier than room temperature, soft icing. I use an offset spatula and put my 1/4 on half. Then I run a dowel down the center through both into the bottom cake board. I use the thick boards for stacked cakes. Tada! Easy peasy. Border it out and decorate both. HTH.

If it's a one-layer 12x18 and a one-layer 9x13 you don't need any supports... it's just like any other 4" tall layer cake. If it isn't going to be cut like a layer cake (through both layers), you might consider using a cardboard under the 9x13 so that each sheet cake can be cut separately into 2" x 2" x 2" servings.