From your description it appears that the property should have originally been titled as tenants in common as your intention was to own the securities jointly (assuming you also have a promissory note or other documents indicating the loan and your joint intentions) and not as business property. If my interpretation is correct, then transferring the assets to a new account titled jointly as tenants in common would not be considered a gift nor a sale and the transfer is merely to correct the original title registration error. The transfer would not generate any tax reporting (1099-B) by the investment brokerage firm. A tenants in common account would be considered owned 50% by each of you and each tenant's portion would go to his respective heirs upon death. Upon the maturity or sale of the securities, each tenant would be responsible for reporting gain/loss on his respective tax return based on the difference between the sales proceeds and each tenant's cost basis.
http://homebuying.about.com/od/marketfactstrends/qt/0207TinCommon.htm
http://www.shiredirect.com/mortgage-glossary/joint-tenancy-vs-tenants-in-common.html
Enrolled Agent
EA, QPA, CHFC, CEBS, CLU - 29 years experience providing financial advice