Universe sandbox is a good one. I think it costs about $20, but it's definitely worth it. Has some systems preloaded, and lets you set up your own with planets, asteroids, multiple stars, and such. edit:
link edit 2: It appears that it's also on steam.
Also, with the twin planets example, in real life, they'd probably be tidally locked to each other, as Pluto and Charon are (the closest real example to a double planet).
Edit:
And here's how I plan on keeping invaders from getting through my jump points. Two at each gate should be enough.
Osa class Jump Point Defence Base 23,600 tons 3249 Crew 6342.5 BP TCS 472 TH 896 EM 0
1898 km/s Armour 10-73 Shields 0-0 Sensors 10/16/0/0 Damage Control Rating 19 PPV 271.52
Maint Life 6.55 Years MSP 9191 AFR 234% IFR 3.3% 1YR 370 5YR 5550 Max Repair 437 MSP
Inertial Confinement Fusion Drive E7.2/224/224 (4) Power 224 Fuel Use 72% Signature 224 Armour 0 Exp 30%
Fuel Capacity 1,250,000 Litres Range 132.4 billion km (807 days at full power)
Quad 15cm C6 X-Ray Laser Turret (2x4) Range 320,000km TS: 32000 km/s Power 24-24 RM 7 ROF 5 6 6 6 6 6 6 6 5 4 4
60cm C8 Plasma Carronade (12) Range 320,000km TS: 8000 km/s Power 96-8 RM 1 ROF 60 96 48 32 24 19 16 13 12 10 9
Fire Control S06 160-12000 (2) Max Range: 320,000 km TS: 12000 km/s 97 94 91 88 84 81 78 75 72 69
Fire Control S04 40-32000 (2) Max Range: 80,000 km TS: 32000 km/s 88 75 62 50 38 25 12 0 0 0
Solid-core Anti-matter Power Plant Technology PB-.5/8 (18) Total Power Output 144 Armour 0 Exp 5%
Active Search Sensor MR11-R1 (1) GPS 48 Range 11.5m km Resolution 1
Active Search Sensor MR51-R20 (1) GPS 960 Range 51.5m km Resolution 20
Thermal Sensor TH2-10 (1) Sensitivity 10 Detect Sig Strength 1000: 10m km
EM Detection Sensor EM2-16 (2) Sensitivity 16 Detect Sig Strength 1000: 16m km
ECCM-5 (2) This design is classed as a Military Vessel for maintenance purposes