Both exist, and the shape tells you which is which. Long flexible designs are made for two people to share. Shorter, firmer ones with a bulb at one end are made to be worn — one end is held internally while the other penetrates. Check the shape and length before buying, because the two are not interchangeable in practice.

Longer gives more room to adjust position and distance; shorter puts partners closer together, which some couples want and others find limiting. If you are unsure, a longer flexible design is the more forgiving choice since you can use as much or as little of it as suits the position.

Considerably. A softer shaft is comfortable but absorbs movement, so less pressure transfers from one end to the other. A firmer one holds its shape when both people move, which makes it easier to control but less forgiving on angle. Dual-density designs sit between the two.

A strapless strap-on is a double-ended toy shaped specifically for retention, with a bulb designed to be held by the wearer, and often vibration to help. A general double-ended dildo is symmetrical and made for sharing rather than wearing. See Strapless Strap-Ons if wearing is the goal.

Only some can. A harness needs a base that seats in the O-ring, and a symmetrical double-ended shaft usually has no base at all. Check the individual product before assuming. If harness use is the aim, harness-compatible dildos are made for it.