You can set up a linked server and do what you want. Look either at BOL or do a google search. In my last job we did what you want to do. It involved linking an MSSQL db and an Oracle db. We used a linked server and a trigger to move data both ways.
The MSSQL stores index info for images, and the Oracle was used to perform AP functions. We would scan invoices and po's. During the scanning process I would set up dummy values for the type of PO. Every 30 minutes, a trigger would kick off, sending data to the Oracle side. The po's would be processed, including the real po number, etc. Another trigger then would replace the dummy info on the MSSQL side with the real info. Generally it worked well.