That slow browsing is probably the fault of IS and not openweb, they're having some speed issues at the moment. You should keep the openweb account and get yourself a webafrica prepaid account (R75 per gb) which carries over to the next month. Use that to open steam and then use CS through openweb. If you want both connections open at the same time you need to install raspppoe and routesentry to route local and international traffic through the different accounts.
So does this mean you can get your ADSL modem to authenticate with 2 different accounts at once, on one line? Didn't think that was possible.