You don't say which country/cultural context the applications are for. I'm familiar with the UK setting in particular. Here, there is no problem using the same research proposal for different applications -- it's even expected.
There is the additional wrinkle in the situation you describe that one potential supervisor might be investing significantly in the development of your proposal. If you were to go to a different institution, having made good use of that contact's input, then it would also be reasonable for that contact to be a teensy bit hacked off -- unless there were mitigating circumstances (e.g. appreciable difference in funding levels).
Generally, however, it will be expected by potential supervisors that applicants will be testing the waters at more than one institution (they ought to advise applicants to do this in any case!). Transparency should not be a problem in most cases (i.e., inform your contact that you are, in fact, applying elsewhere too). Except for prima donnas, that shouldn't be an issue.