Burkholderia contaminans PSB-A and Burkholderia ambifaria PSB-B were isolated from button mushroom bed to estimate their phosphate solubility. The phosphate-solubilizing abilities of these strains were assessed by measuring the phosphorus content in a single and co-inoculation medium for 7 days. The co-inoculation of these two strains released the highest content of soluble phosphorus (166.3 μg mL-1) into the medium, followed by single inoculation of B. contaminans PSB-A (143.73 μg mL-1) and B. ambifaria PSB-B (127.1 μg mL-1). The highest pH reduction, organic acid production, and glucose consumption were also observed in the co-inoculation medium. According to the plant growth promotion bioassay, coinoculation enhanced the growth of romaine lettuce much more than the single inoculation (20.4% for leaf widths and 16.6% for root lengths). Although no significant difference was noted between single and co-inoculation of bacterial strains in terms of phosphorous release and plant growth, co-inoculation of PSB may have a beneficial effect on crop growth due to a synergistic effect between the strains.