Structure of a dissertation with two (fairly) disjoint topics
My recommendation is to consider "parts" as an auxiliary notion. The thesis should make sense to the reader even if he ignores the division into parts. 

That's fairly doable with your proposed solution, it just needs clever organization of introductory and preliminary texts.

Actually, I don't think you can do much better if your 4 chapters are too long to become 2 chapters.



The only alternative I can think of is the following:

Ch.1 - Introduction (discussing general structure &amp; motivation)

Part I - Theme 1

Ch.2 - theme 1 - deeper introduction &amp; preliminaries
Ch.3 - theme 1 - topic 1
Ch.4 - theme 1 - topic 2 

Part II - Theme 2
Ch.5 - theme 2 - deeper introduction &amp; preliminaries
Ch.6 - theme 2 - topic 1
Ch.7 - theme 2 - topic 2

Ch.8 - Conclusion

Appendices and References


Also, as a last remark, if you realize there's nothing to put into the conclusions, I'm quite sure you can create the thesis without them, but better ask your supervisor.

Actually, have you discussed this with your supervisor?