Provided you have access to their specific sealable bags, yes, you could theoretically do that. But don't you think they'd look at you twice if you asked for a second bag, with seal strip still in place? Why on earth would you want a second bag that was not sealed yet?
The seal strips on such bags are usually glued such that once sealed, you would have to tamper with the bag to open it (i.e. cut it open, rip it open). The US has similar security, although generally the shops hang on to the bag with alcohol and tobacco and deliver it to the gate so you don't even have time to tamper.
S.