The safe voltage varies depending on who you ask. Personally for starts, i'd say stick to 1.15 or less.
Follow these steps:
1) Install MSI afterburner off your MSI cd. Do not install the beta version floating around the web! It has a few stability issues (the 2.2 beta).
2) Go in settings, enable voltage control and voltage monitoring. Close AB and restart.
3) Voltage. Again that depends on who you ask, but generally i'd recommend 1.1V or less for your 1st overclock. Thats roughly +100 in AB. Try that first, as 1.1 is safe. That said, many people have gone higher than 1.1, but 1.1 is my comfort point - I can get 950 GPu clock easily at 1.1v on my lightning.
4) Generally MSI lightnings overclock to 900+ easily. The best method to do this, is to start overclocking in 10 mhz clock / 10mV voltage increments until you find something stable. To test stability, download the heaven dx11 benchmark program - test a new overclock, run heaven for about 30 minutes to check stability. If you see flickering or artifacts, that means you're not stable.
5) You can overclock or overvolt them independantly. In your settings panel in MSI AB, you can select GPU 1 / GPU 2 depending on which SLI card you want to overclock. If you want them both at the same clock speed, click the "synchronize different GPU's" checkbox.
6) If you're not stable either 1) lower your gpu speed or 2) increase voltage
and then run heaven again. +rep if helped