What Suki said
As long as their guidelines do no say no simultaneous submissions, you are free to sub elsewhere. And should imo. Some pubs take years to get back to you!
Anyway, IF you get an offer from the second pub, you shoot the first an email. Hi, have had an expression of interest in other quarters. Can I ask if you are willing to make an offer? (or you know, good. Polite, professional etc) If I could have an answer within (X time)..
Then shoot pub 2 an email asking for X time to consider the offer.
No reputable pub will bat an eye. Provided their guidelines did not say no sim subs.
There is a risk in this, of course -- by pushing for an answer you may nudge them towards a no. Which is why you only do it when you already have another offer in hand...However, if they liked it and were contemplating offering anyway, it may play in your favour. Swings and roundabouts.