Take a look at the Schematics I have in the Technical Reference Section regarding Buck / Boost Transformer connections using a typical Isolated Transformer, which is located at: Buck/Boost/Choke + Polarity Test drawings for Transformers
In a nutshell, what you will be doing is creating an Autotransformer out of an Isolated Transformer. The connection scheme will have the coil arrangements in a Series Additive setup.
A quick description:
[*] Connect the "Split Coils" on each side in Series (for the "Higher" listed Voltage). This means connect H2 and H3 together, also hook X2 and X3 together. Do this on both Transformers.
[*] Connect the "H4" end of the Primary coil to the "X1" end of the Secondary coil.
[*] Feed this side with one Ungrounded Conductor - for example, this would be Phase C on the "Right Side" Transformer - for the "Left Side" Transformer Phase A would be used.
[*] Connect an "Output" Conductor to "X4" for the Load side connection (Transformer Output).
[*] Connect "H1" to an Ungrounded Conductor which will be "Common" for this arrangement - in this example use Phase B for the common Ungrounded Conductor.
[*] For an Open Delta arrangement, the above would be done using Two Identical Transformers.
[*] For a Wye Connection, Three Identical Transformers would be used, and the Common Conductor would be the Grounded Conductor. Primary coils would be setup in Parallel (lower Voltage).
Use Ed's example drawing above [the Open Delta one] for applying these figures to.
Using the 7.5 KVA 120/240 VAC x 16/32 VAC Transformers, you will end up with 240 VAC output [208 VAC + 32 VAC], but with a maximum continuous KVA available output of 37.5 KVA per Transformer.
Let us know if you have further questions and how things turn out.