I want to get another 980 founders edition for sli, now my PSU only has 2 6 pin connectors, i heard there was a certain cable conversion like something to 6 pin, what is it called. My cpu is an i5-8400 and 650 watt psu
I don't like using adapters, there's no easy way to be sure about balancing load distribution and also the connector you are adapting from may not be rated for that much power. Also a 980 can peak to about 280W under some conditions so a 650W PSU may be a little light to be safe. Do it if you want but it's a risk.
I sold my 980 FE and got a second hand 1070 when the prices started dropping, not much of an upgrade but all I needed. I'd personally recommend getting a 1070ti/1080/1080ti over going SLI with two 980s
Do not EVER use a power supply with adapters in order to run a single card that needs more than it has, or dual cards.
As mentioned, if the PSU was capable enough to run a specific card or cards, it would already have the connectors. Even some power supplies that have the correct connectors may not be good enough for a given setup, but if it doesn't have them at all, then it's a really bad idea.
If your PSU has only two 6 pin connectors, it's ABSOLUTELY not good enough, nor high enough capacity, to run GTX 980 SLI.
980 sli recommended is 750w, but that's also taking into consideration OC and quality of the psu. Any good 650w will have at least 2x6pin and 2x6+2pin, more than capable of 980 sli except for the highest tier OC 980s which have 2x8pin pcie and can exceed 280w power consumption.
Grandfather's have been harping on using the right tool for the job since before you were born, you wouldn't use a screwdriver to drive a nail, and good luck with a 10lb sledgehammer. Psus are no different. You want sli, get a psu that's designed for it in a size capable of it.