In my games I allow people to "cross-class", however I have my own rules for it.  To give you an idea, here's how it would work:

    Trax is a Solo with Combat Sense +8, he decides he needs to make himself more diversified and begins looking into starting shop work (Techie), as soon as he is set on it, and spends the points required to get a Jury Rig of +1, his Combat Sense drops to +7.  Trax is NOT forbidden from raising his Combat Sense ever again, like a lot of systems make say.  However, every time his Jury Rig goes up, his Combat Sense goes down by the same increment  (thus, plus one to Jury Rig, negative one to Combat Sense).  Trax can purchase ranks in Combat Sense without affecting his Jury Rig.

    Characters are limited to ONLY 2 Roles at a time.  To pick up a new role the character must study, be active in, or associate in/with the new role for at least a month before gaining the first rank.  (Changing your career is rarely an overnight thing).

SPECIAL RULES:

    If the character has Combat Sense, then picks up Sneak, when both reach equal levels, the player has the option of combining the two into Combat Sneak at 2/3 the current value.  This is a definite benefit if the character is at the time thinking about trying to pick up an addition Role.

    Characters who choose Fixer as their new role can make a Very Difficult Streetwise check to pump their current contacts, associates, etc. for a way in to the biz.  If successful, The new Fixer starts off with a Streetdeal of 2 instead of 1.  If unsuccessful, the character starts at 1, and likely isn't off to a good start.  If fumbled, the character is in deep dreg (GM's discretion as to what the actual consequences are).  Either way getting into the biz is going to cost the player.

    Attempting to become a recognized Nomad can be more difficult than it seems, depending on the Nomad pack.  Joining a nomad pack, typically, will require a persuasion roll (GM determines difficulty, though I would also recommend that (s)he makes the player role play it as well...and remember, many nomad packs are paranoid, or at least wary, of statics).  Players with "Local Nomad Pack Befriends you" or similar lifepath events, can add in their bonus to their persuasion roll, as long as it is with the appropriate nomad pack, if it is a pack that has an antagonistic relationship with the nomad pack that the character befriended; first roll against the player's REP, if the pack has heard of him, assume they know about his connection with the antagonistic pack, and subtract the bonus from the persuasion roll.  Fumbles may mean a complete falling out between the player and the pack.  If a player is attempting to gain acceptance with a pack that another player is associated with, and the nomad player is attempting to help his companion, have both players roll persuasion checks, the nomad getting to use his Family score as a bonus to his roll.

    Any character looking to become a Solo, would likely do well looking at the Solo Lifecycle.

    These are just some guidelines, the full details are up to you.  If you have any questions, suggestions, or ideas, please e-mail me, and let me know!

    Now, I know some people want an explanation better than "it's for game balance", so here it is:
    The concentration, time, and energy it takes to raise up a special ability (which is at the core for every role in the game) takes time away from the previous profession, and remember, these are professions.  "The more you learn, the less you know" definitely applies here, old skills get rusty as you concentrate on new ones.