Volume 1 will contain all the questions and Model Solutions
Volume 2 will contain all the detailed solutions
If it isn't too expensive, it doesn't seem all that bad considering that it's two books and covering the entire course (and also assuming that goes quite in-depth). These are probably minor details that you'd probably be thinking about later, but I think two things that would be neat are:
If the detailed solutions book were, to a small extent, readable on their own (e.g. reading the solutions as a set of notes / textbook almost). I haven't actually seen your physics book yet, so I'm not sure if you already did this, but I think you could cater for that if you just had a sentence at the start of each solution which noted which topic / concept it was assessing. While it's probably not a good idea to read the answers beforehand, but it would allow the second book to be used in a slightly different manner once you've already done the questions.
Direct page references in the model solutions to the detailed solutions for the question would make the books less 'fiddly', in terms of having to find where in the other book the answers are. e.g.
Model Solution
Detailed solutions can be found in Vol-II, Page-132
2a. solution
2b solution
Detailed solutions can be found in Vol-II, Page-133
2c.
3a.