Both bolts should be able to be torqued up to the recommended value (typically around 50 inch pounds or 5-6 N.m. Less if your dealing with a carbon fiber steerer. Check with manufacturer.). They should also be tightened down together, keeping a similar gap between the two sides of stem the bolts are bringing closer.
While it's not likely the stem and handle bar unit would become totally disengaged from the fork steerer tube under normal, leisurely riding conditions, neither is it safe counting on one bolt and far less surface area of the steerer tube that's bound by it. Replacement makes the most sense considering the (usually) reasonable cost of alloy threadless stems similar to yours.
An alternative to helicoil (the cost of which would equal a double digit percent of a new stem) or the through-bolt w/ washer & nut. Would be to tap new threads into the receiving hole and use a new M-bolt with the equivalent diameter & thread pitch of the tap. If the current bolt is an M5 (diameter of the threaded part of your current fastener), tap in M6 x 1.00 threads and use an M6 socket head cap screw for the new fastener. This also is spendy if you don't have access to a tap and die kit. I mention it because I've noticed these kits showing up more and more in home shops and tool closets.
Using grease or other thread prep on bicycle fasteners is an important part of repairing or installation. It prevents corrosion, helps the threads not become galled and allows easier achievement of the spec'd torque value while minimizing risk of damage to the part, the fastener and the threading.