SCPRedMage |
The only way to be proficient with using a bastard sword two-handed but NOT one-handed is to have proficiency with ALL martial weapons but NOT have a specific proficiency with bastard swords; gaining a specific proficiency for a specific weapon means you are proficient with them, in all their intended forms of use.
I suppose if something explicitly called out that it granted proficiency with bastards swords, but only when used two-handed, it would be an exception, but I've yet to see anything that does so.