3 studies of interest:

Study 1
http://europepmc.org/abstract/med/8531617
– 2 Groups
– Flexibility training only
– Strength training and Flexibility training combined

Flexibility only group increase range of motion and flexibility more than the combined group

The results indicate that the FO group increased its range of motion in shoulder abduction to a significantly greater extent than the SF group (P < 0.001), and none of the changes in range of motion for the SF group was significantly different than the changes in the control group.

Study 2
http://ajs.sagepub.com/content/13/4/263.short
– Contract-relax stretching technique, is more effective than traditional ballistic stretching

Forty-seven male subjects were randomly assigned to 4 different groups. Three groups of 10 subjects trained three times a week with a modified contract-relax (CR) method for improving muscle flexibility. Seventeen sub jects trained during the same time with a traditional ballistic stretch (BS) method. After 30 days (14 training sessions) the latter group switched over to the CR method. The results showed that the CR method was significantly better than the BS method for improving muscle flexibility in the four different, bilateral muscle groups studied.

Study 3
Just to contradict the first study:

To test the hypothesis that increases in muscle strength and flexibility are developed by specific training programs, 43 healthy young adults were tested before and after 4 different interventions conducted twice a week for 12 weeks: (a) resistance training only (n = 13); (b) flexibility training only (n = 11); (c) resistance and flexibility training (n = 9); and (d) no intervention (n = 10). There was no change in either strength or flexibility in the control group (p > 0.05).
Resistance training improved muscle strength either alone (114%; effect size = 0.53; p < 0.001) or in combination with flexibility training (116%; effect size = 0.66; p = 0.032), but did not change flexibility (p = 0.610).

Flexibility increased with specific training alone (133%; p < 0.001) or in combination with resistance training (118%; p < 0.001).

In conclusion, in young, healthy subjects, resistance training alone did not increase flexibility, but resistance training did not interfere with the increase in joint range of motion during flexibility training. These results support the concept that specific training should be employed in order to increase either muscle strength or flexibility.