First off, underwriters are unlikely to support new projects more than double the size of prior work. Additionally, they generally expect the financial analysis of the last company financial year-end financial statement to show adequate levels of working capital for such projects. (Read Secrets of Bonding # 4 for a complete explanation regarding working capital calculations.).
The aggregate capacity is generally double the "single" amount although there may be cases where a limited program consists of a single and aggregate for the same amount. This would mean the underwriters only wish to support one project at a time with no overlap.
As far as the ability to bid on multiple projects while performing other work, the solution is to have an aggregate amount that is a multiple of the single, for example $1 million single/ $2 million aggregate (referred to as "one over two").
To decide if the aggregate is enough, first determine if it consists of bonded work only or all projects. This varies by underwriter. It is reasonable and likely they will say "the aggregate includes all work, bonded and unbonded." This approach takes all the contractor's obligations into consideration, everything that may tax financial and human resources and therefore affect the bonded work.
Knowing when current bonded projects will complete can be useful. Underwriters may support bids knowing that the start of the new project will be after the completion of a current bonded job. This is a slightly creative way of stretching the capacity with a view toward the future. Some underwriters will exercise this flexibility.
That's the predicament of being a multi-talented, creative and aspiring trailblazer - to discern when to follow the "tried-and-true" materials (because reinventing the wheel is frequently stupid) and when to burst out of the box, trust your guts and follow your own creativity.