While I have not used the share, I do have some experience with the Feeldoe Slim and a double strap style harness. It is certainly possible to hold the Feeldoe in and have pretty good control over it, IF the wearer has pretty good vaginal muscles. You can use a harness with the Feeldoe and the Share, and my experience says that the Feeldoe works pretty well with a harness, only you don't have as much control over the angle of the dildo because of it being stuck in the harness.
A harness and strap on dildos would lend a pretty good threshold of variety as you have a lot more options available than you do when buying a double ended dildo. Also if dual stimulation is important (strap on sex is always stimulating though

) I might suggest a harness that allows for two dildos to be used in it, one facing outward to penetrate the partner, one facing inward to penetrate the wearer. Outlaw leather makes some good harnesses for this, just be warned they are expensive.
I use my Feeldoe for Pegging with my husband, and purchased that particular one because of the firmer silicone and the included vibrating bullet. Looking back on it, I wish I had purchased the Syd harness and two dildos instead (or maybe in addition), as we found the Feeldoe Slim to be rather short. Don't get me wrong, we really love it. The shortness of it just makes some positions more difficult and it lends to a pretty shallow penetration. The Feeldoe More is an inch longer, and I am yearning to give that hot red beast a try

Best of luck, and hope you find something perfect for you and your girlfriend!