Hi Jaye, Kimberly here from landscape-solutions-for-you.
Thank you for asking the question on your two apple trees. Apple trees need full sun to grow. They also do better in an open area, away from woods. I would also recommend away from your septic tank. Spetic tanks aren't installed too deep, and many tree roots will spread possibly in and around the septic tank. Cable wires are usually buried anywhere from two to four inches deep.
One of the things I recommend for residential owners is to plant dwarf or small apple trees. Dig your hole double the diameter of the root ball and two feet deep. A good thing to do if possible, is to find out how deep the nursery grew your apple trees. You should consider digging the same depth as they did.
If you plant your two apple trees in an open area, away from you septic tank, and also not in line with hitting your cable wire, you should be fine. Don't fertize when you first plant your trees. Wait until their root systems have had a chance to get established.
One way to measure the root depth, is to take the width of a mature apple specie that you are planting as this is approximently how deep the root system will be.
I hope this has helped some, and thanks again for asking the question on your apple trees. If you need more help or have any other questions, feel free to contact me here at landscape solutions for you. Have a great day!
Kimberly