According to the official Pokémon GO Trainer guidelines players are, shockingly, not allowed to cheat.
"Methods of cheating, unfortunately, are limited only by cheaters’ imaginations, but include at a minimum the following:... sharing accounts...".
The Pokémon GO Terms of Service states that:
"...our Trainer Guidelines... govern your use of our Services."
In other words, breach of the Guidelines can give rise to a reason to apply the banhammer. Given that they can "terminate your access to and use of the Services, at [their] sole discretion, at any time and without notice to you", there is a good chance that you could get banned for this.
For the time being I wouldn't suggest account sharing. Just wait until trading starts, which at the current rate should be some time in the next 5 years...