Are you:

Hip

See me once, with two joints, already half high

Hip joints, half way up your body.

See me twice, as a part of the toast that the people will cry

HipHip, Hooray!

See me once, at concerts, but not Bob Marley's

Concerts are hip and happening, but Bob Marley's are hiphop, so being hip is seen too much?

See me twice, aren't these some cheerful parties?

HipHip, Hooray! (this time, at a party)

Are you:

Hip

See me once, with two joints, already half high

Hip joints, half way up your body.

See me twice, as a part of the toast that the people will cry

HipHip, Hooray!

See me once, at concerts, but not Bob Marley's

you see hip at HipHop concerts. But Bob Marley does Reggae

See me twice, aren't these some cheerful parties?

HipHip, Hooray! (this time, at a party)

Are you:

Hip

See me once, with two joints, already half high

Hip joints, half way up your body.

See me twice, as a part of the toast that the people will cry

HipHip, Hooray!

See me once, at concerts, but not Bob Marley's

Hip concerts tend to be modern/popular, I wouldn't class Bob Marley's music as hip.

See me twice, aren't these some cheerful parties?

HipHip, Hooray! (this time, at a party)

Are you:

Hip

See me once, with two joints, already half high

Hip joints, half way up your body.

See me twice, as a part of the toast that the people will cry

HipHip, Hooray!

See me once, at concerts, but not Bob Marley's

Concerts are hip and happening, but Bob Marley's are hiphop, so being hip is seen too much?

See me twice, aren't these some cheerful parties?

HipHip, Hooray! (this time, at a party)