Split link chain fitting
First thing, I'd agree with Kidder, if you can get the thing rivetted, then 100% peace of mind is assured.
But if you still go down the split link route, I've used the following method without any problems in the past.
Assemble the link complete, but without the split circlip.
Using a pair of solid long nose pliers, squeeze the two side plates together to compress the "O" rings in the chain. This should also reveal the circlip locating grooves on the pins.
Holding the side plates together, fit the CLOSED end only of the circlip over the pin that is the leading pin when travelling, and ensure that it locates in the groove, then release the side plates. If the clip is in correctly, then the plates won't come apart.
Now slide/rotate the clip until it is next to the second pin, and using a very small screwdriver, lift the edge up just enough to jump over the top of the second pin. At this point, the closed end of the clip will have moved off the front pin but should still retain the side plates.
Once the circlip is engaged over both pins, using the long nose pliers again, squeeze the leading edge of the clip and the first pin together, and the clip should slide and lock into it's correct location.
Make sure the closed end of the clip faces the normal direction of chain travel.
If this don't help ...
rivett the beggar!!