Personally, I wouldn't bother with 1H proficiency, since swinging a longsword from horseback without a shield in your hand uses 2H proficiency. I don't actually hold my shield up that often on horseback, either. The passive defense from a shield against missiles hitting you in the back is really nice as well. With some decent armor, arrows and bolts will glance off your back when you're riding away pretty frequently. (That large damage reduction from the shield plus the large negative movement modifier is enough to nullify a lot of the rangers out there)
Of course, you can always pull the shield off your back if there's lots of archers aiming at you, and 1 proficiency swings are still viable, especially when attacking someone who's unaware of you. Anyway, that's what I would do if I wanted to use a bastard weapon.