ok, now I see where you are going.
ok, lets say your internet connection has an upload speed of 512Kbit/s (upload is the opposite of download, when you send a mail, that mail gets uploaded to your ISP's server). So what happens now is that you send a mail and that is uploaded with 512Kbit/s or at least as fast as it can go, let's say you are sending a big file that takes one minute to send. If you send two mails with equal big files it takes you one minute each after another, so you need two minutes to send those files. Still with me?

You want simultaneous connections. So lets say you are sending two of those big files at the same time. Both connections have to share that 512Kbit/s connection, so each one is only getting 256Kbit/s speed. So the speed is halved for each send, which makes each file take two minutes to send. Since they are sent at the same time it takes you two minutes alltogether to send those two mails...
You see where I'm aiming at? It will take you the same time regardless whether the mails are sent after each other or at the same time. Theoretically at least. In the actual implementation I can see that you probably will get a bit of speed gain if the smtp server of your ISP is not as fast as your upload speed.
Similar to what happens when you use so called download accelerators.
But then again, most smtp servers are pretty fast and will not suffer from this.
How to get more speed/faster internet connection? Change ISP or get a faster service from your ISP. If you are using an old analog modem, get cable or dsl, if you got cable or dsl, see if you can get faster deal with your isp.
I hope you understood what I wrote. I tried to make it understandable but I fear that I probably just confused you
