Since database connections seems to be compile time, and you have to set A4GL_SQLTYPE in order to compile against database X - I wonder how you can open concurrent connections.. I cannot find any documentation on how to do this - but according to http://aubit4gl.sourceforge.net/aubit4gldoc/manual/html/features_summary.html it maybe that you have to use ODBC connections to each instead of native??
It seems that "open session" and "use session" can be used to dynamically switch at runtime. But compile time seems to be the kicker.. especially if you use "define x like table.column" in 4gl, unless it allows "define x like db.table.column" or searches both databases.