Newly forming protoplanets are expected to create cavities and substructures in young, gas-rich protoplanetary disks$^1-3$, but they are difficult to detect as they could be confused with disk features affected by advanced image analysis techniques$^4,5$. Recently, a planet was discovered inside the gap of the transitional disk of the T Tauri star PDS 70$^6,7$. Here, we report on the detection of strong Hα emission from two distinct locations in the PDS 70 system, one corresponding to the previously discovered planet PDS 70 b, which confirms the earlier Hα detection$^8$, and another located close to the outer edge of the gap, coinciding with a previously identified bright dust spot in the disk and with a small opening in a ring of molecular emission$^6,7,9$. We identify this second Hα peak as a second protoplanet in the PDS 70 system. The Hα emission spectra of both protoplanets indicate ongoing accretion onto the protoplanets$^10,11$, which appear to be near a 2:1 mean motion resonance. Our observations show that adaptive-optics- assisted, medium-resolution integral field spectroscopy with MUSE$^12$ targeting accretion signatures will be a powerful way to trace ongoing planet formation in transitional disks at different stages of their evolution. Finding more young planetary systems in mean motion resonance would give credibility to the Grand Tack hypothesis in which Jupiter and Saturn migrated in a resonance orbit during the early formation period of our Solar System$^13$.