Couple of unexplained pieces of information...
Two installs using the same database? A ZC 1.5.7 install expects the database being accessed to have a different "configuration" than any previous version. (basically 1.5.7 has modified the database as compared to say 1.5.6 or any previous version.) So... to be sure we are talking about the same thing, do you mean that your 1.5.7 is pointing to exactly the same database (including DB_PREFIX) as the earlier version? If so, one of the two is not going to work correctly. Either the 1.5.7 because it hasn't been updated for 1.5.7 or the earlier version because the database has been updated and information *may* not be located where expected. As far as 1.5.7 to previous versions this is likely to be seen where ez-pages are concerned because those have been made multilingual and required some rearranging of information, tables, etc...
Now, if you mean that two stores reference the same database but both have different DB_PREFIX values, then yes that is possible. Usually the issue with "changing" the includes/configure.php file is that it hasn't been made writeable before attempting to save changes.
Now, as far as the blank screen without debug log, that is possible if the problem is really a php parsing error instead of some of the other "generic" issues. I don't have a great example, but I know that sometimes if I am not paying complete attention, I might leave out a backslash at the end of one of the paths, or in some sort of copy and paste a back quote gets used instead of a single quote or some other random mistake.
You could post the content of your includes/configure.php changing pertinent information to "protect the innocent". Exact letters are not necessarily as important to those here as is the format, quotes, slashes, and the like. Please obscure/obfuscate password/paths of concern, but be consistent with whatever you do and don't make up a line/quoted text ending that isn't in your version.
All that said though... if the standard upgrade by rebuild process were used, then the software would create the necessary configure files and there wouldn't be an issue such as this.
Could also help by posting both the "old" and the "new" includes/configure.php files and explaining why a difference exists between the two (e.g. This path is different because it points to the sub-domain of the test install.)
Also, it may be of assistance (maybe) to identify in a password type field if there are special characters involved (single quote, greater than, less than, ampersand, etc...)
To assist really I think it comes down to providing more information, even if it all seems to blur together at this point.
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