Flexible energy resources can help balance the power grid by providing ancillary services. However, the balancing potential of most types of resources is restricted by physical constraints such as the size of their energy buffer, power ramp-rate limits, or control delays. Using the example of secondary frequency regulation, this paper shows how the flexibility of energy resources can be exploited more efficiently by considering multiple resources with complementary physical properties and controlling them in a coordinated way. To this end, optimal adjustable control policies are computed based on robust optimization. Our problem formulation takes into account power ramp-rate limits explicitly and accurately models the different timescales and lead times of the energy and reserve markets. Simulations demonstrate that aggregating select resources can lead to a synergy effect that frees up additional regulation capacity in the range of 18%-1190% compared with the amount of reserves the systems could provide individually. This synergy effect can unlock the regulation potential of energy resources with control delays or restrictive energy or ramp-rate constraints.