I'm assuming the beer is already carbonated. In that case.... You want to start with both kegs pressurized to a little bit higher than the equilibrium pressure of the beer (that is, pressure needed to maintain your carbonation level at the current temperature. What you find on a carbonation chart, or your serving pressure.) Then gently bleed pressure from the receiving keg to start and maintain flow, but don't let it drop below the pressure of equilibrium. This will prevent any gas breakout and foaming. It's the same principle as counterpressure bottling.