M98 Controller providing OJT described a loss of separation event when the Developmental turned a second departure; cutting off a preceding departure aircraft. Corrective vectors were issued too late to maintain separation.

Narrative

Air Carrier X departed MSP from Runway 30L on a 260 heading from Tower. At approximately 4 miles the Developmental turned Air Carrier X to a 120 heading direct to DOKTR; a fix on the RST3 SID. Meanwhile; Air Carrier Y departed MSP from Runway 30L on a 260 heading approximately 3 miles in trail of Air Carrier X. The Developmental proceeded to turn Air Carrier Y at approximately 3 miles to a 210 heading; so cutting off Air Carrier X. Air Carrier Y had initial separation from the Tower but by turning the second aircraft too early (at approximately 3 miles) the Developmental turned him in front of and at Air Carrier X; versus behind the aircraft. They were not going to hit; but the Developmental did not ensure positive separation. I told the Developmental that there was not going to be separation and he/she turned Air Carrier Y to a 270 heading to pull the aircraft apart; but it was too late and there was approximately 2.8 miles and approximately 300 feet. We discussed the need to maintain the initial 3 miles separation until you ensure another form of separation; such as degree divergence. Recommendation; as the Trainer; I would not let a Developmental make the transmission to turn the second aircraft at the first aircraft without ensuring positive separation prior to the turn; during the turn; and after the turn. I would not wait to have the Developmental fix it; I just wouldn't let him/her do it again. We also discussed the contributing factors that set up the conflict in the first place. We discussed two problem areas 1) an effective scan 2) poor control judgment to turn an aircraft inside and/or at another aircraft where you cut yourself off.