Magic smoke will be forced out of the small radio perhaps even a loud magic noise, too. (-:
While a reputable electrician would probably not make this particular mistake, it's possible for a future modification or rearrangement of breakers in the panel to inadvertently move one or both individual breakers (especially in older work) so that they both end up on the same 120V leg of the panel - which is improper when a shared neutral is involved. If the split circuit is installed that way, the shared unbalanced load could exceed the rating of the wire.
Maybe some NEC expert can tell us when the use of a double-pole breaker became a code requirement for Edison circuits because I know it wasn't always that way (or that way in 1988). My kitchen had an Edison circuit
*without* a tied breaker until I rewired it with separate runs. Without the tied breakers or a double pole breaker that kills power to both phases, it is quite possible for someone to be shocked while working on the circuit, since the neutral wire of the supposedly "dead" circuit could be carrying current from the sister "live" circuit. DAMHIKT. (-:There's so little benefit, IMHO, that the risk of using Edison circuits today doesn't seem worth it. If voltage drop is a problem, use a larger diameter wire. If you can't afford the extra wire, check the car's seat cushions for spare change or sell some blood. (-:
Besides, the last time we had this discussion I believe I pointed out that because "Romex" 12/2 w/G and 14/2 w/G is so widely used, it can often be had on sale for much less than 12/3 or 14/3 w/G, thus totally negating any real savings in wire costs.
Same problem with the 2 pole GFCIs needed to protect an Edison circuit. I can always find a good sale price on the single circuit GFCIs. I don't recall ever seeing a double pole unit on sale. I do recall when I looked at
2P GFCIs they cost way more than 2 separate 1P GFCIs. I recall paying a little more than $10 a pop for 20A Leviton GFCIs just a while back when I ran a new sump pump circuit.The 2P units Google shows are in the $80-$110 range. So it looks like the Edison circuit is going to end up costing way more than two comparable single branches, at least if you don't pay list price for your components.
I don't see the tradeoffs being worth it but obviously some people do. For me it violates the KISS rule.