The best way to level all of the Syndicates at once is to finish your current Priority contract, then at Daily Reset, switch allegiances. You will now get 50% bonuses to contracts from that faction and additional Dark Matter Ingots. Then do whatever you can to work on all 3 faction bounties at once during one event. Tomorrow, do the same thing. The only progress you will ever lose if the Priority contracts you currently have are not finished.