Multiple traders from same pool account

Does anyone know of a discount brokerage company in Canada that enables multiple traders to trade simultaneously from the same account. My brother and I are both active day traders and would like to pool our money in one account and trade from it.? We presently use esignal software and are both accredited investors.

 

What happens if you buy 100 eMinis and your brother short-sells 100 eMinis without telling you?

 

Actually he trades five different stocks that I do not trade.