H1BUser24:
It is legally permissible to join an H1B petitioner based upon the receipt notice. As long as the petition is ultimately approved, then there is no issue with this strategy.
However, if the new H1B petition is ultimately denied months later (after you have left the original company and they have withdrawn the H1B petition), this would generally leave one "out of status" and potentially "unlawfully present", which can have serious consequences.
Sometimes there is no way around this issue, particularly when premium processing is unavailable. However, there are risks to doing so.
This is ultimately a personal decision, based on the relative merits of each employer - if your current employment is becoming insecure, this is definitely a factor to consider.