I will admit this idea had not occurred to me, due to my poor knowledge of BIP_32. Most end-users I don't think use two wallets as I have suggested several times -- and that is a serious security problem. In fact, most of them don't even take the simple precaution of backing up important info like their Electrum seed or their PGP keys or generating a separate revocation certificate in case of key compromise or loss.

One of the problems in trying to get new features into this is I am not the developer of the plugin that is being used. I knew virtually nothing about bitcoin before working on Electrum except that it used ECC and that the proof of work was used to do away with a central authority for processing transactions. I spent many hours trying to get my output from Electrum to match byte for byte bitcoind's output before I realized that ECDSA isn't always deterministic! My goal at the outset was simply to be able to sign a transaction so I could complete it as a user of the site without having to download the entire blockchain. Since the devs of the site turned it into a plugin, I have only supplied patches on their forums for problems users have had. If I had known about that javascript-based site, I would've likely never worked on this at all. Of course, if I knew what I know now I would've likely used pybtctools to accomplish my goal.