The good news is that you do not have to. Since budgets are set at the campaign level, most campaigns are already a hybrid of prospecting & remarketing users. Unless you have a clear strategy in place to only show a particular ad to past visitors, for example, to give a discount code or special offer that you only want past visitors to see, then breaking them out into another campaign will be the better solution. In this case, you should set a separate budget for your remarketing ads.