All accounts? I have not done it, but it looks to be possible with MC.NET. Same is also possible with NinjaTrader which does have a DLL interface.
I think MC .NET cannot do this,even with programming. Would love to hear from someone from MC to comment on this.
To be clear, I'm not suggesting using the standard DOM Trader with MC, Ninja or even TradeStation. All of these products allow programmatic input of Account and Symbol. NT with the DLL interface, MC .Net by exposing the Trade Manager, and TS with the use of Order Objects. All three would require front-end modifications or new development. I have not done it, and would need to dig further, but it does seem possible at first glance.
Are the accounts your accounts or the accounts of others that have granted you access? I suspect that will make a difference. I suggest exploring IB. If the accounts are your accounts then it is just sub-accounts. If the accounts belong to other people (less than 15 accounts) then possibly the friends and family model.
I am talking based on their (MC) programming capability (incapability) due to design limitations. They are basically a single thread structure, cannot mitigate among the difference positions in a single account openned by different threads. IBKR so far is the best bet (yes, programming wise).