It depends on the site of the planter. If it is too big then you should go for one planter as it will come in between of the front door and if its not that big then go for two separate planters.
It must be difficult as sliding greenhouse doors doesn't have standard size. Many doors have wheels fitted in the door frame. Just measure the size properly and get the best sliding door.
Push some slates in the ground along the fence line. This will keep weeds away.