Section 53. A veteran of the Indian wars who is employed jointly by two municipalities, if incapacitated for active duty, may be retired from active service by the joint action of the governing bodies thereof, whether the city council or selectmen, as the case may be, at one half the rate of the average compensation paid to him during the two years immediately preceding his retirement, one half of the said retiring compensation to be paid by each of the municipalities employing him; provided, that no veteran shall be retired under this section unless he has been in the service of the two municipalities, as aforesaid, for a period of not less than ten years preceding the time of his retirement. But a period of employment by either one of said municipalities immediately preceding his employment by both jointly shall be reckoned as a part of the said ten years.