Your question is rather vague, are u an agent or an ordinary citizen? I am kinda bored, so i will try to answer u from all hypothetical angles so that whatever it is u are failing to ask clearly, u will be answered.
Scenario 1) Assuming you are an ordinary Zimbabwean, it is very legal to use both telecash and ecocash, as far as i undestand zim law, u are allowed , infact its your right to choose whichever company suits your fancy, so as long as u have a telecel line and econet line, ( as well as the cash to transact) u can register on both networks.
Scenario 2) you are an agent, well if by legal u mean the jurisdiction of zimbabwe u are alllowed to deal with both companies, but wait there is a little catch, there is something called terms and conditions whenever u sign contracts, now for telecel it appears they dont give a tinker's damn if u are also on ecocash or not, but hell, econet seems to be freaked out by the idea of its agents doing telecash business too, so they claim that they are gonna close down on their agents who are double dipping, so in essence in the eyes of zim law is legal if u are a dealer or customer but for econet if u are their agent, its illegal and they will kick you out. Now i am not making up these things, for reference sake check a story that appeared on one news website where econet allegedly claimed that telecel is pollution and they wont allow their ecocash network to be tainted by it . here go to http://www.iharare.co.zw/telecel-is-pollution-says-econet-as-it-threatens-to-close-down-on-its-ecocash-agents-registering-with-telecash/