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I would first see if the place in which you bought it would take it back, as most should. If not, there may be little you can do. While you could string them together, there is no guarantee that it will work. Split stems will also be an invitation to pests or disease, so just to be safe I would treat with neem oil. Unless you could get two individual plants with roots, dividing it won't do much good either. In the meantime, you could just keep a watch on it. If one of the halves looks weaker or more unhealthy than the other, you could simply remove it.

Mandevilla favor humid conditions. If your plant is exposed to the sun, the soil may be too hot. Try misting/spraying the leaves with water in the mornings and keep the soil moist (NOT wet). You may also want to consider moving your plant (if possible) to an area that doesn't receive direct sunlight for extended periods of time.

Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm

This article will help: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm

There are a number of reasons why the leaves may turn yellow. One might be overwatering. Too much water causes yellowing of the leaves and mandevilla do not like overly wet feet. Another possibility could be too little water. If this is the case, the yellow leaves would be curling up as well. Mealy bugs can occasionally affect these plants, causing them to become weak and leading to yellow foliage. Finally, plant stress due to a change in location, temp., etc. can lead to yellow leaves. Mandevilla favor humid conditions. If your plant is exposed to the sun, the soil may be too hot. Try misting/spraying the leaves with water in the mornings and keep the soil moist (NOT wet). You may also want to consider moving your plant (if possible) to an area that doesn't receive direct sunlight for extended periods of time.

Most likely to much Nitrogen, this will cause lush greens and little flowers.

You can add Phosphorous. Bone Meal is a good way to do this.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/bone-meal-fertilizer.htm