Should I get a RWD now or wait for dual motor?
Range is only one factor in this decision. A 3 LR has comparable range to an S 100D. And if all you want is range, you can save $$$ by going with a 3 vs. an S.
The S will have faster acceleration, though for most drivers the difference probably isn't significant enough to be a factor.
The S has more storage area with the rear hatchback vs. the trunk in the 3.
We have a 2012 S P85 (RWD) and a 2017 S 100D, and for typical driving conditions, the difference in handling between the two cars really isn't noticeable. At least in the S, the dual motor configuration uses slightly less energy at highway speeds; if that's true, a 3 LR D could have more range than an S 100D.
Another factor to consider is the $7500 tax credit. Since Tesla hasn't said anything about this, it's likely they haven't hit the 200K trigger yet. Though it seems likely that could happen sometime this quarter. And if it does, the full $7500 credit will be available only through the end of September. If you order an S or 3 LR today, you should get it before October - and get the full $7500 credit.
However, if you decide to hold out and wait for an 3 LR D, since there could be many others also waiting for that configuration, it's possible your delivery date could be pushed out to October - and you'd lose half of the EV tax credit.
For us, we've been able to configure for 2 months (which will replace our S P85), and have been waiting for the white interior option. If we can also get dual motor at the same time, we'll order that, even if it means we might risk losing half of the $7500 credit.